U.S. Pat. Nos. 4,721,500, 4,609,140, 4,721,499, 5,088,640, 5,326,020, 4,832,676 and 4,606,496 describe processes and apparatus for forming food service paperboard container products such as paper trays, plates and the like from a paperboard blank, and food service paperboard container products formed from such processes and apparatus. The patents describe that the paperboard blank is formed into the desired paperboard container product through use of a pressing apparatus that applies heat and pressure to the paperboard blank.
The apparatus typically used in the disclosed process includes an upper die and a lower die. In practice, both the lower die and the upper die are heated by a heating mechanism in the form of one or more electrically resistive ring heaters. Ring heaters are commonly used because they are relatively inexpensive Typically, the lower die is provided with a pair of nested ring heaters or a single ring heater, while the upper die is outfitted with either a pair of nested ring heaters or a single ring heater, depending upon the particular product being formed. Quite often, the heat requirements related to the way in which the paperboard container products are formed necessitate that at least two ring heaters be provided in both the upper die and the lower die.
FIG. 1 illustrates a pair of nested ring heaters similar to those used in the apparatus for press forming paperboard blanks into paperboard container products. As can be seen from FIG. 1, the pair of nested ring heaters includes an annular outer ring heater 60 and an annular inner ring heater 62, with the inner ring heater 62 being nested within the outer ring heater 60. The pair of nested ring heaters 60, 62 is fitted inside a cavity in the lower die. As mentioned above, the upper die can be provided with a similar pair of nested ring heaters, or it can be outfitted with only a single ring heater, depending upon the requirements of a particular system.
Ring heaters can be purchased as off-the-shelf items from a suitable supplier. However, to produce food service paperboard articles in the manner described in the aforementioned patents, significantly high temperatures are required and this necessitates that the ring heaters possess a very high wattage. Thus, for these applications, it is typically necessary to special order the ring heaters having the desired wattage, as well as the desired size, voltage and/or terminations.
As can be further seen from FIG. 1, the outer ring heater 60 includes a pair of terminals 70 and the inner ring heater 62 also includes a pair of terminals 72. Typically, the terminals 70 on the outer ring heater 60 are connected to the power source, and an electrical connection is then made between the pair of terminals 70 on the outer ring heater 60 and the pair of terminals 72 on the inner ring heater 62.
It has been found that a cold spot (lower temperatures) typically exists in the region of the terminals 70, 72 due to the terminal connections which means that a cold spot will also exist in the die in the region of the pressing surface corresponding to the locations of the terminals 70, 72. Thus, to avoid a cold spot in the die and provide generally uniform heating around the entire circumference of the heater mechanism defined by the pair of ring heaters 60, 62, it is desirable that the pair of terminals 70 on the outer ring heater 60 be offset 180° with respect to the pair of terminals 72 on the inner ring heater 62. In practice, this has been found to be somewhat problematic in that the wire that connects the pair of terminals 70 on the outer ring heater 60 to the pair of terminals 72 on the inner ring heater 62 can run excessively hot in the enclosed die cavities, resulting in wire insulation degradation and failure due to the wires shorting to ground. Thus, it is common in practice to position the inner and outer ring heaters 60, 62 in the manner illustrated in FIG. 1 so that the terminals 70, 72 can be connected together with an electrical buss bar or the like. This connection is not susceptible to the same type of failure mentioned above. However, this orientation of the two ring heaters 60, 62 creates a cold spot at the place where the terminals 70, 72 are located. This thus creates a temperature differential around the circumference of the die. By virtue of this arrangement of the nested ring heaters 60, 62, it has been found that a temperature differential of up to approximately 30° F.-40° F. can exist between the portion of the die forming surface corresponding to the location of the terminals 70, 72 and the diametrically opposite portion of the die forming surface that is offset 180°. From the standpoint of carrying out the method described in the aforementioned patents, this temperature differential can be quite significant and rather problematic.
It has also been found that the wires used to connect the outer ring heater 60 to the power source have a tendency to break. It is believed that this is due at least in part to the fact that the wire is connected directly to the ring heater 60 and passes thru part of the hot enclosed die cavity resulting in wire degradation. Further, the cycling/movement of the press apparatus can result in wire breakage due to repetitive flexing. The relatively high temperature at which the ring heaters are operated may also contribute to this problem.
A further problem associated with the use of ring heaters in the context of the pressing apparatus described in the aforementioned patents for press forming a paperboard blank into a food serving paperboard container product involves the significant discrepancy between the wattage ratings of the ring heaters and the wattage needed to perform the pressing operation. To carry out the press forming operation described in the aforementioned patents, a relatively high die surface temperature in the range of about 280° F.-400° F. is typically required. Further, the press forming apparatus must be capable of high speed production. The wattage needed to carry out the process described in the aforementioned patents (i.e., to achieve the necessary temperature on the die surfaces) at the high speed production required for commercial practice is much higher than the wattage for which the ring heaters 60, 62 are rated. The pressing apparatus is typically designed in a way that only permits the use of ring heaters of a certain size and so it is not a truly viable option to simply use larger ring heaters rated at a higher wattage. Given the limitations imposed by the current design of the pressing apparatus, the ring heaters that are capable of being used in the pressing apparatus have power wattage ratings on the order of 300-1200 watts. However, during operation of the pressing apparatus in the manner necessary for carrying out the process described in the aforementioned patents, the ring heaters are typically run at a much higher wattage, on the order of 1500 watts-5000 watts. Operating the ring heaters at levels such as these that greatly exceed the wattage ratings of the heaters significantly reduces the life of the ring heaters.
The ring heaters 60, 62 illustrated in FIG. 1 also suffer from certain disadvantages and drawbacks by virtue of the way in which they are constructed. FIG. 2A illustrates the outer ring heater 60 in cross-section. As seen in FIG. 2A, the ring heater 60 includes an outer sheath defined by a generally U-shaped portion 74 and a plate-like cover 76. The U-shaped portion 74 and the cover 76 are commonly made of steel, aluminized steel or incoloy alloy stainless steel. The U-shaped portion 74 includes a bottom wall 84 and a pair of upstanding sidewalls 82 whose upper ends are bent inwardly towards one another. The cover 76 is then secured to these inwardly bent ends of the upstanding sidewalls 82. Positioned within the sheath 74, 76 is a coiled wire 78 that is surrounded by a refractory material 80. The wire 78 is commonly made of nichrome steel to obtain the required electrical resistance. The refractory material 80 is commonly magnesium oxide powder which is compressed during ring heater production.
The difficulties associated with this ring heater construction is that the magnesium oxide refractory material 80 has an affinity for water. During use of the ring heater as the ring heater is heating up, the Incoloy sheath 74, 76 can expand and distorts so that small spaces are created in the area where the cover 76 is secured to the inwardly bent ends of the sidewalls 82. Water is thus able to infiltrate into the magnesium oxide refractory material 80 during ring heater cool down. As the ring heater becomes hot during subsequent heating, the water that has infiltrated into the magnesium oxide refractory material will become heated, thus creating steam pressure that distorts the configuration of the Incoloy sheath. Ideally, it is preferred that the bottom wall 84 be as flat as possible to provide the largest contact area with the facing bottom surface of the recess in the die. If water infiltrates the magnesium oxide refractory material and ultimately is transformed into steam pressure that distorts the sheath, it has been found that the bottom wall 84′ of the Incoloy sheath tends to distort and take on a curved configuration as shown in FIG. 2B. This curved bottom wall 84′ creates a non-flat heating surface which means a significant loss of contact area with the die, thus causing inadequate non-uniform heating. Also, this can significantly reduce the conductive heat transfer to the die. This is part of the reason why the ring heaters need to be operated at a wattage significantly higher than the wattage rating of the ring heaters.
A further difficulty associated with the aforementioned distortion of the ring heater is that it places further strain on the operational rating of the ring heater. Typically, the upper and lower dies in which the pair of nested ring heaters are positioned include respective thermocouple probes that measure the temperature near the surfaces of the upper and lower dies to determine the operating parameters of the respective ring heaters. When the thermocouple detects that the temperatures are too low, the ring heaters are turned on at full wattage for time proportioned periods regulated by a temperature controller. When the bottom wall 84′ of the ring heater distorts in the manner illustrated in FIG. 2B, heat is not effectively transferred to the die and so the thermocouple senses that the die heating surfaces are not hot enough. This causes the ring heaters to be run at full wattage and higher temperatures for longer time periods, thus creating further operational problems and significantly reducing the operating life of the ring heaters.
The ring heaters typically used in the pressing apparatus for carrying out the process described in the aforementioned patents are typically rated at 240 volts. If it is necessary to use a higher voltage, the length of wire wound in the ring heater can be increased. However, because of the size limitations associated with the construction of the lower and upper dies, it is difficult to use a larger ring heater having an increased amount of wound wire. The other alternative for achieving the higher voltage is to use thinner wire or wire having a reduced diameter and higher electrical resistance. Unfortunately, it has been found that this makes the wire particularly susceptible to breakage or other damage due to the operating conditions associated with pressing apparatus, namely the repetitive pounding and impacting, and the high temperatures.
From the foregoing, it is apparent that the use of ring heaters as the heat source for heating the pressing surfaces of a pressing apparatus that is designed to press form paperboard blanks into paperboard container products presents a variety of disadvantages and drawbacks that have a tendency to reduce the useful life of the ring heaters. This thus requires that they be replaced on an excessively frequent basis. The need for repairing or replacing damaged ring heaters and/or ring heater wires also presents a further significant problem relating to productivity. As noted above, each die set consisting of an upper die and a lower die typically includes three or four ring heaters, two ring heaters in the lower die and one or two ring heaters in the upper die. Typically, there are 4-7 die sets per press so that 4-7 paperboard container products can be produced during each cycle of the press. Thus, there are approximately 12-28 ring heaters per press. If one ring heater or ring heater wire breaks down or fails, the entire press has to be stopped so that the ring heater or wire can be replaced. This of course shuts down all of the other die sets in the press which can have a significant adverse impact on productivity. If the press is operating at full capacity, shutting down the press for even a relatively short period of time necessarily results in a significant reduction in production. If it is necessary to increase production at another facility (or at the same facility assuming the affected press is not operating at full capacity) to make up for the press down-time, it may be necessary to incur added expense (e.g., overtime for the press operators), thus further increasing the costs associated with making paperboard container products.
In light of the foregoing, a need exists for a pressing apparatus that is able to press form a paperboard blank into a food service paperboard container without the disadvantages and drawbacks associated with current apparatus.
A need also exists for such a pressing apparatus that is not susceptible to the excessive amount of operational difficulties and manufacturing downtime as current pressing apparatus.
It would also be desirable to provide a pressing apparatus that provides a more efficient heat transfer to the die to negate the need for operating the heater at an excessively high wattage.